The enzyme, pyruvate oxidase, has been characterized by negative staining and transmission electron microscopy. The molecule consists of 4 tightly packed subunits and resembles a sphere flattened at both ends. It is 11.5 nm high and 12.4 nm in diameter. One of the regulators of the complement "cascade", C3b-C4b inactivator co-factor, is a 10 to the 6th power dalton protein that closely resembles IgM in its structure.